Conjugation of a polysaccharide to a carrier protein can effectively make that polysaceharide more immunogenic. Tetanus toxoid has been used for decades in this capacity as a carrier, and its safety profile has been established, at least in the context of past uses.
The structural gene for tetanus toxin has been cloned and sequenced. Fairweather et al., J. Bacteriol. 165: 21-27 (1986); Fairweather et al., Nuci. Acid Res. 14: 7809-7812 (1986). These studies have confirmed the structure of tetanus toxin as a 150 kD protein comprising 1315 amino acids. Fragment C, which constitutes the binding portion of native tetanus toxin, is a 52 kD polypeptide generated by papain cleavage of the toxin and corresponds to the 451 amino acids at the C-terminus. See FIG. 1.
Tetanus toxoid has been found to contain 2 to 3 universal T-cell epitopes. Demotz et al., J. Immunol 142: 394-402 (1989). This feature makes tetanus toxoid highly effective in humans. Fragment C of the toxoid has been shown to be nontoxic. This fragment also contains at least one of the universal immunogenic T cell epitopes recognized by primed donors. Valmori et al., J. Immunol 149:717-2 1 (1992); Panina-Bordignon et al., Eur. J. Immunol. 19: 2237 (1989).
Capsular polysaccharides (CP) conjugate vaccines targeting a variety of bacterial infections are currently under development and clinical evaluation. The inclusion of multiple CP serotypes combined in a single injection is currently under study. The combination of CP conjugate vaccines into a single multivalent injection, however, can result in competition among the different components and adversely affect the immunogenicity of any individual conjugate. Fattom et al., Vaccine 17:126-33 (1999).
Tetanus toxoid is finding increased use in polysaccharide vaccines. There is now concern arising that the vaccinated population will be over exposed to Tetanus, with the risk of inducing tolerance and/or hypersensitivity throughout the population. For example, injection of mice with an immunogenic dose of carrier followed by immunization with hapten-carrier conjugate selectively suppresses antihapten antibody response. This carrier-induced epitopic suppression may be related to the induction of carrier-specifics T cells which in turn could inhibit selectively antihapten response. Epitopic suppression may induced through the expansion of the clones specific for the carrier epitopes and antigenic competition between hapten and carrier epitopes. Schutze et al., J. Immunol. 37: 2635-40 (1989). In humans, it has been demonstrated that prior immunity against a carrier protein modulates the serological response to synthetic conjugate vaccines. Di John et al., Lancet 2 (8677):1415-8 (1989). Barrington et al. (Infect. & Immun. 61: 432-8, 1993) have shown that epitopic suppression of antibody response to Haemophilus influenzae type b conjugate vaccine by preimmunization with vaccine components was observed. More recently, Burrage et al. (Infect. & Immun. 70: 4946-54, 2002) have shown some epitopic suppression of antibody response to meningococcal C conjugate vaccine by preimmunization with the tetanus carrier protein. In mice, epitopic suppression to the antibody response of pneumococcal and meningococcal polysaceharide-tetanus conjugates was observed after high doses of carrier priming with tetanus toxoid (Peeters et al. Infect & Immun 59: 3504-10, 1991). Due to these potential adverse consequences, tetanus toxoid should no longer be administered as it has in the past, and therefore improved carriers are needed.